STiMi, a wireless channel transmission technology oriented to the mobile multimedia broadcasting design, and it is the core in the structure of the China Mobile Multimedia Broadcasting (CMMB) system independently developed by China. STiMi technology fully considers the characteristics of mobile multimedia broadcasting services and uses the most advanced channel error correction coding and orthogonal frequency division multiplexing (OFDM) to improve the anti-interference capability and the support for mobility, in accordance with the high requirement for receiving sensitivity, mobility and battery supply of hand-held devices.
The basic principle of the OFDM is to transform the high-speed serial data into multi-channel relatively low-speed parallel data and to modulate different carriers. The parallel transmission mechanism extremely extends the pulse width of symbols as well as improves the anti-multipath-fading performance. Meanwhile, the spectra in different subcarriers are overlapped, but the spectra are orthogonal with each other in a whole symbol period, which not only guarantees that the signal can be recovered without distortion at the receiving end, but also largely improves the spectrum utilization rate.
In the OFDM system, the receiver needs to perform frame synchronization capture and OFDM symbol synchronization capture, and then can carry out demodulation correctly. STiMi technology creatively uses the time-domain spread spectrum beacon for the synchronization capture, which has features such as short synchronization capture time, strong anti-carrier frequency offset and anti-channel multi-path delay extension capabilities. This method largely reduces the synchronization time needed from the user starting up the receiver to normal reception. Especially in the situations of emergent broadcasting, this method guarantees that the user can carry out reception rapidly and reliably.
The time domain and frequency domain responses of the wireless channel are time-varying, and the frequency domain selective fading introduced by multi-path is also inconsistent in different subcarriers, thus OFDM symbols might be unevenly distorted in different subcarriers. Therefore, the method of channel estimation must be used to estimate the time domain and frequency domain responses of the channels, so as to correct and recover the received data. STiMi uses the pilot technology, which not only guarantees reliable channel estimation and equalization in complicated wireless transmission conditions, but also decreases the hardware complexity of the demodulation module, thus it is beneficial to the chip implementation.
In order to conveniently synchronize the receiver, the CMMB mobile TV standards specifically design a sync signal. The subcarrier interval of the sync signal is 4.8828125 kHz, which is two times of that of the data signal. It is composed of a PN sequence in frequency domain, and two identical signal segments in time domain.
Fractional frequency offset might cause the interference among the OFDM subcarriers, and after it is corrected, the interference between the adjacent subcarriers caused by it is basically eliminated, and the OFDM multi-carrier signal can be correctly demodulated. Compared with the demodulation result after the frequency synchronization, the demodulation result at this time has an even-numbered offset of subcarriers, called integer frequency offset. The integer frequency offset uses the good correlation property of the PN sequence to estimate in frequency domain.
Since the integer frequency offset (n*Δf) will result in periodically cyclical shift n of the subcarrier symbol data sequence after the FFT operation, the data cannot be demodulated correctly, thus it needs to estimate and then to compensate; the integer frequency offset estimation is implemented by estimating the cyclical shift amount k of the synchronization symbol data sequence after the FFT operation, meanwhile, in order to guarantee the orthogonality between the subcarriers in the processing process of the FFT operation, it needs to carry out fractional frequency offset compensation processing for data before the FFT. In the following, synchronization symbol is used to carry out the integer frequency offset estimation.
The frequency offset estimation module mainly estimates the fractional frequency offset for the synchronization data, and performs FFT operation on the data on the basis of the fractional frequency offset compensation, and then carries out the integer frequency offset estimation for the data. The main sub-modules included are : the initialization synchronization differential sequence module, the initialization trigonometric function table, the fractional frequency offset estimation, the fractional frequency offset compensation, the FFT operation and the integer frequency offset estimation.
In general conditions, the frequency offset estimation module uses software or hardware to implement 2048-point FFT. The CMMB protocol specifies, however, that each OFDM symbol comprises 4096 sample points, therefore a 4096-point FFT needs to be performed. During the implementation, generally both FFTs are respectively performed, as shown in FIG. 1. This method needs two FFT modules, which causes resource waste and has low utilization rate, meanwhile, it might increase difficulty in digital signal processing (DSP) or software control, and also consume the number of the cycles.